In the development of the traditional type of papermaking machine known as a Fourdrinier machine, a moving endless wire carrying paper stock is passed over a series of conditioning stations which have been subjected to a variety of improvements throughout the years to make the papermaking process more efficient, inexpensive and generally improved. The moving wire encounters two major areas for conditioning the wet material carried by it. First the wire passes over a series of rollers, stationary foils or doctor blades and second over a series of suction boxes. At both stations, the general objective is to remove water or other fluid from the paper stock, in other words drainage of water therefrom. Suction can also be applied at the location of the stationary foils or rollers as well as at the suction box station.
The general trend has been toward the use of a stationary doctor blade or drainage foil rather than rollers and various improvements in that type of structure have appeared through the years. The improvements have been primarily in changes in configuration for the foil, replacement portions for the foil and means for mounting and replacing the foil, as well as the provision of improved wear surfaces for engagement with the moving foil of the papermaking machine. These criteria have been found to be of great importance in this type of structure since the forming wire moves at a very high rate of speed.
Also, this wire with the wet sheet of stock material is quite heavy and the stationary foil must support this weighted moving wire as water is doctored or drained from the wire.
The surface of the foil in engagement with the moving wire is subjected to high wear conditions and must be replaced frequently. Thus, ease of replacement is important and improvements in this area have been made so that the machine will be out of operation for the minimum possible time while foils are being replaced. Additionally, different types of wear surfaces which are harder and subject to less deteriation due to wear over longer periods of time have been developed. Due to economic considerations and replacement considerations, these harder surfaces are often formed as part of the foil, the part in direct contact with the moving wire.
After the moving wire and transported paper stock material passes the location of the stationary foil or rolls, it comes into contact with a series of suction boxes which further draw water from the paper stock or substrate on the moving wire. The suction boxes are arranged so that they have an upper wear surface with apertures therein. The wire passes over and is supported by the upper wear surface of the suction box while vacuum draws water through the apertures in the suction box cover. Thus, in general, the wear criteria dealing with the nature of the surface of the stationary foil which is in contact with the moving wire is also applicable to the suction box cover or surface which is in similar direct engagement with the moving wire. Similar developments in attempting to improve wear surface have occured in regard to suction box covers and so called forming surfaces as shown in British Pat. No. 1,403,158.
One area of development in regard to improving the wear surface of the stationary drainage foils and the suction box covers deals with the use of a ceramic wear surface which has a number of the desired characteristics for the surface as discussed above. Three developments relating to suction box covers which specifically discuss the use of ceramics are U.S. Pat. Nos. 3,067,816; 3,250,671; and 3,351,524.
A serious difficulty that arises when dealing with ceramics is that manufacture of ceramics in large sections creates cost problems in respect to both manufacture and installation. Furthermore, in the past, attempts at development of ceramic wear surfaces of smaller component parts or smaller ceramic pieces mounted in some fashion together with remaining structures have resulted in increased manufacturing and installation costs as well as difficulties in operation. This is particularly true in respect to the tendency of one or more of the smaller ceramic components to be displaced or disoriented with respect to the others during fabrication of the drainage device or in use of the drainage device. This is a problem particularly where the high speed and heavy wire is being continuously passed over the surface such as in the common type of Fourdrinier papermaking machine.
Accordingly, in the continuous development of improved drainage devices such as foils and suction box covers there is still room for improvement in order to satisfy all of the desired criteria for that type of structure when dealing with papermaking machinery and other similar types of machinery.